The invention relates to electro-optical systems used to perform extremely accurate measurement of changes in either length or optical path length, e.g., interferometry systems. More particularly, the invention relates to an apparatus for use with an interferometry system in which the apparatus transforms a single frequency, linearly polarized laser beam into a beam with two frequency components that are orthogonally polarized.
The use of optical interferometry to measure changes in either length, distance, or optical path length has grown significantly due not only to technological advances in lasers, photosensors, and microelectronics but also to an ever increasing demand for high precision, high accuracy measurements [cf. N. Bobroff, "Recent advances in displacement measuring interferometry," Meas. Sci. Technol., 4(9), 907-926 (1993)]. The prior art interferometers can be generally categorized into two types based on the signal processing technique used, i.e., either homodyne or heterodyne. The interferometers based on the heterodyne technique are generally preferred because (1) they are insensitive to low frequency drift and noise and (2) they can more readily have their resolution extended. Within the heterodyne type of interferometers, of particular interest are the ones based on the use of two optical frequencies.
In the prior art two-optical frequency heterodyne interferometers, the two optical frequencies are produced by one of the following techniques: (1) use of a Zeeman split laser, see for example, Bagley et al., U.S. Pat. No. 3,458,259, issued Jul. 29, 1969; G. Bouwhuis, "Interferometrie Mit Gaslasers," Ned. T. Natuurk, 34, 225-232 (Aug. 1968); Bagley et al., U.S. Pat. No. 3,656,853, issued Apr. 18, 1972; and H. Matsumoto, "Recent interferometric measurements using stabilized lasers," Precision Engineering, 6(2), 87-94 (1984); (2) use of a pair of acousto-optical Bragg cells, see for example, Y. Ohtsuka and K. Itoh, "Two-frequency Laser Interferometer for Small Displacement Measurements in a Low Frequency Range," Applied Optics, 18(2), 219-224 (1979); N. Massie et al., "Measuring Laser Flow Fields With a 64-Channel Heterodyne Interferometer," Applied Optics, 22(14), 2141-2151 (1983); Y. Ohtsuka and M. Tsubokawa, "Dynamic Two-frequency Interferometry for Small Displacement Measurements," Optics and Laser Technology, 16, 25-29 (1984); H. Matsumoto, ibid.; P. Dirksen, et al., U.S. Pat. No. 5,485,272, issued Jan. 16, 1996; N. A. Riza and M. M. K. Howlader, "Acousto-optic system for the generation and control of tunable low-frequency signals," Opt. Eng., 35(4), 920-925 (1996); (3) use of a single acousto-optic Bragg cell, see for example, G. E. Sommargren, commonly owned U.S. Pat. No. 4,684,828, issued Aug. 4, 1987; G. E. Sommargren, commonly owned U.S. Pat. No. 4,687,958, issued Aug. 18, 1987; P. Dirksen, et al., ibid.; or (4) use of two longitudinal modes of a randomly polarized HeNe laser, see for example, J. B. Ferguson and R. H. Morris, "Single Mode Collapse in 6328 .ANG. HeNe Lasers," Applied Optics, 17(18), 2924-2929 (1978).
As for the prior art use of a Zeeman split laser to produce the two optical frequencies, this approach is only applicable to certain lasers (e.g., HeNe) and limits the frequency difference between the two optical frequencies to about 2 MHz. This imposes a limit on the maximum rate of change of the length or optical length being measured. In addition, the available power from a Zeeman split laser is less than 500 microwatts, which can be a serious limitation when one laser source must be used for the measurement of multiple axes, such as three to six axes.
The acousto-optical modulator with a single acousto-optical Bragg cell of Sommargren, commonly owned U.S. Pat. No. 4,684,828 and of Dirksen, et al., ibid., and the acousto-optical modulator with two acousto-optical Bragg cells of Dirksen, et al., ibid., are based on normal Bragg diffraction in both non birefringent and birefringent Bragg cells. The normal Bragg diffraction generates a diffracted beam wherein the state of linear polarization of the diffracted beam is the same state of linear polarization as the incident, undiffracted beam. However, the objectives of the heterodyne interfergmetry are usually best served when the two optical beam components from an acousto-optical modulator are frequency shifted one with respect to the other, orthogonally polarized, and collinear. The process of converting the output beam components generated by a normal Bragg diffraction acousto-optical modulator, i.e., two non collinear beams in the same linear polarization state into two collinear beams in orthogonally polarized beams, has had an efficiency significantly less than 100%.
Accompanying the increasing demand for improved high precision, high accuracy distance measurements is a demand to increase the number of axes being measured with distance measuring interferometry. The demand to increase the number of axes being measured translates to either increasing the number of laser source-acousto-optical modulator units, increasing the power of the laser source, and/or increasing the conversion efficiency with respect to power of the two frequency heterodyne source. An increase in the conversion efficiency is clearly an attractive option from a commercial point of view.